Generally speaking, a cordless telephone includes a handset coupled via radio connection with a base station. The base station is usually connected by wire to a traditional Public Switched Telephone Network (PSTN) or an Integrated Services Digital Network (ISDN). The development of new cordless standards which are based upon digital technology provide a broad spectrum of applications. Exemplary cordless applications include wireless Private Automatic Branch Exchange (PABX), wireless Local Area Network (LAN), Telepoint, and Radio Local Loop. Cordless standards include Digital Enhanced Cordless Telecommunications (DECT), GSM, PHS, AMPS, IS54 or IS95. The digital cordless telephones represent a valid alternative to cellular phones in densely populated areas.
DECT is a cordless standard defined as a Multicarrier (MC), Time Division Multiple Access (TDMA)/Time Duplex Division (TDD) system. Time is divided in the DECT standard into frames of 10 ms. Each frame is divided into 24 full slots. The standard also allows for half slots and double slots of data.
In order to be able to support multiple channels, a DECT base station compresses and transmits 10 ms of speech during one full slot. This means that 10 ms of speech are actually sent over the radio in 416 .mu.s. Every active connection makes use of two slots, one for receiving and one for transmitting. For example, if the slots in a DECT frame are numbered from 0 to 23, the first 12 slots (0-11) are used for transmission from the base station to the handset and the remaining slots are used for handset to base station transmission. A base station transmitting to a given handset in slot N will always receive from this handset in slot N plus 12, or in other words, half a frame later. Accordingly, a DECT base station is able to support up to 12 active voice connections at the same time.
The total number of bits within a conventional DECT slot is 480.With 24 slots and a 10 ms frame, a gross bit rate of 1.152 Mbits/s is provided. Once the DECT slot has been formatted, it is transmitted using one of 10 radio frequencies specified within the DECT standard. For example, the frequency band assigned to DECT in Europe is between 1,880 and 1,900 Mhz, with a spacing of 1.728 Khz between adjacent frequencies. The transmission frequency for each channel is chosen dynamically based upon a Radio Signal Strength Indication (RSSI). Each active slot in the DECT frame may be transmitted and received on any of the 10 frequencies.